Pluripotency of Induced Pluripotent Stem Cells
Volume 11, Issue 5, October 2013, Pages 299303
Special Issue: Induced Pluripotent Stem Cells
Edited By Qi Zhou
Induced pluripotent stem (iPS) cells can be generated by forced expression of four pluripotency factors in somatic cells. This has received much attention in recent years since it may offer us a promising donor cell source for cell transplantation therapy. There has been great progress in iPS cell research in the past few years. However, several issues need to be further addressed in the near future before the clinical application of iPS cells, like the immunogenicity of iPS cells, the variability of differentiation potential and most importantly tumor formation of the iPS derivative cells. Here, we review recent progress in research into the pluripotency of iPS cells.
Induced pluripotent stem (iPS) cells can be derived from mouse somatic cells via the ectopic expression of four defined factors, Oct4, Sox2, Klf4 and c-Myc (also known as Yamanaka factors) [1]. The mouse iPS cells express pluripotency markers and both X chromosomes are reactivated, allowing differentiation into various cell types of three germ layers when injected into a blastocyst. iPS technology makes reprogramming much easier [2]and[3] in comparison to early reprogramming methods such as somatic cell nuclear transfer (SCNT) [4]and[5], iPS technology also circumvents the ethical problems arising from the use of human oocytes. In addition, the generation of patient-specific iPS cells could be used to screen new drugs [6]and[7]. However, there are currently several limitations in applying iPS cells clinically. Efficiency of converting somatic cells to iPS cells is still very low. In particular, only approximately 0.1% to 1% of somatic cells experience changes at the transcriptional level and finally become pluripotent stem cells when non-integration approaches are used [8]. Moreover, compared to embryonic stem (ES) cells, the developmental potential and differentiation capacity of iPS cells is significantly reduced and there is increased variability among all iPS cell lines [9]. In mice, only small proportions of these cells are fully reprogrammed based on the most stringent tetraploid complementation assay for evaluating pluripotency [10], [11], [12]and[13]. Therefore, it is necessary to establish a strict molecular standard system to distinguish fully reprogrammed iPS cells from those partially reprogrammed, as we currently lack suitable in vivo pluripotency tests for human iPS cells.
In this review, we mainly focus on recent progress on rodent, non-human primate and human iPS cells, and point out some key questions which need to be addressed in the near future, such as the pluripotency level of human iPS cells and the establishment of a new standard to assess the pluripotency level of human iPS cells.
Takahashi and Yamanaka reprogrammed mouse embryonic fibroblasts by the ectopic expression of four reprogramming factors using retroviral vectors, and finally produced iPS cells which resemble ES cells [1]. This original iPS reprogramming approach used viral vectors, including retrovirus and lentivirus which possess high reprogramming efficiency [14]and[15]. The genome may be mutated by integrating other gene sequences, thus raising concerns on the safety issue. In addition, the insertion of oncogenes, like c-Myc, increases the risk of tumor formation [16]and[17]. Subsequently, several modified methods were used to obtain much safer iPS cells, for instance, piggyBac transposon [18], adenovirus [19], sendai virus [20], plasmid [21], episomal vectors [22] and minicircle vectors [23]. However, the reprogramming efficiency is significantly decreased and it takes longer to reactivate the key pluripotency markers to achieve full reprogramming. Therefore, efficient generation of non-integrated iPS cells by new approaches may promote their clinical application.
Recent studies have described several reprogramming methods using proteins, RNAs and small-molecule compounds to derive safe iPS cells [24], [25]and[26]. Zhou et al. obtained iPS cells induced by recombination of the proteins of the four Yamanaka factors obtained by fusing the C-terminus of the proteins with poly-arginine (11R) [24]. A recent study reported that mouse and human iPS cells can be efficiently generated by miRNA mediated reprogramming [25]. Miyoshi et al. [26] successfully generated iPS cells by direct transfection of human somatic cells using mature miRNA. iPS cells can also be generated by synthetic RNAs, which bypass the innate response to viruses [27]. Recently, Houet et al. [28] showed that pluripotent stem cells can be generated from mouse somatic cells at an efficiency of 0.2% by using a combination of seven small-molecule compounds. Compared to traditional viral methods, the aforementioned approaches can be used to generate qualified iPS cells (Table 1) without the risk of insertional mutagenesis. Nonetheless, some familiar drawbacks exist, such as a longer and less efficient reprogramming process. In other words, what we need to do next is to optimize non-integration induction systems in order to resolve these drawbacks.